Tuberculosis

TB skin test

06/27/2007

Question:

About 5 years ago I moved to Hawaii and was required to take a 2 step skin test for employment. This was negative. Approximately 5 month`s later, I switched jobs, and was required to take another skin test. This turned out positive (or so I was told). It was red, and raised and I believe measured about 10mm. I was really suprised and about a week later, I had it repeated. This time it was definitly negative. I have had them done every 6 months since then, and it has always been negative. Are false positives common? What would`ve caused this reaction?

Answer:

Tuberculin skin testing is done using PPD, which stands for Purified Protein Derivative. The purified protein is derived from filtrates of culture media that has been used to grow Mycobacterium tuberculosis. It is a sterile (i.e., no living germs) product that contains several antigens that people who are infected with Mycobacterium tuberculosis germs usually react to with a positive result of the skin test. Unfortunately, the PPD test may also be positive when a person is infected with one of the many non-tuberculous Mycobacteria (there are many different types in soil and water, so it's not unusual to be exposed, and people are seldom aware of these infections). We call that a false positive test, and it sounds like you had a false positive PPD.

There is a blood test now available in the U.S., called the Quantiferon-Gold®, and there will soon be a second blood test available, that use antigens that are much more specific for Mycobacterium tuberculosis infection. The results will cross-react with only about 3 of the many environmental Mycobacteria around. If you ever had any question about whether you were infected with Mycobacterium tuberculosis or not, you could have this blood test done.

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